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Abstract

Post-transcriptional regulation of gene expression sits at the core of proteomic complexity; trans-acting factors that regulate RNA localization and translation capacity are thus indispensible. In this thesis, I present studies of the cytoplasmic polyadenylation element binding protein (CPEB), a sequence specific RNA-binding protein important for cell cycle progression and neural synaptic plasticity. I focus on CPEB because the activity of RNA-binding proteins affects the destiny of their mRNA substrates. As presented in Chapter II, CPEB, though mostly cytoplasmic at steady state, shuttles between the nucleus and the cytoplasm. Surprisingly, the RNA recognition motifs are essential for the nuclear localization. CPEB associates with the polyadenylation machinery in both compartments, suggesting it is involved in both nuclear mRNA processing and cytoplasmic translational regulation. Moreover, the nuclear translocalization is critical to relay a tight translation repression on CPE-containing mRNAs. Chapter III focuses on the regulation of CPEB dimerization. CPEB dimerizes through the RNA-binding domains to inhibit its own RNA binding ability in a cell cycle-dependent manner. By dimerizing, CPEB has enhanced binding to protein destruction factors so that robust active degradation occurs in the later cell cycle. The degradation of CPEB is required for translation activation of a subset of mRNAs and cell cycle progression. In addition, dimerization protects cells from being overloaded with excess CPEB. In sum, the localization and dimerization status of CPEB is dynamic and highly regulated; they in turn regulate the activity of CPEB, which results in responsive translation control. These studies provide a strong foundation to decipher CPEB-mediated gene expression.

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